The behavior of a polymer melt with respect to how it processes depends on its previous thermal history, and not just the stress that it sees at the beginning of or during a process. When the polymer melt is slowly deformed, its flow behavior is in the linear (i.e. Newtonian) viscoelastic range, and the Boltzmann superposition principle describes the memory function of the melt, from which the future viscoelastic response can be derived, in particular the melt's viscosity and elasticity. Industrial processing of polymeric melts preferably occurs at high throughput and involves higher strain rates. At strain rates that are sufficiently high, non linear effects such as shear thinning are observed for pseudo-plastic melts, and flow models exist which describe shear-thinning viscosity reduction quite well: a good example is the Carreau equation which calculates the viscosity of a polymeric melt at any temperature T and strain rate:
      η    ⁡          (              T        ,                  γ          .                    )        =                    η        o            ⁡              (        T        )                            (                  1          +                                    (                                                                    η                    o                                    ⁢                                      γ                    .                                                                    τ                  F                                            )                        a                          )                              (                      n            -            1                    )                /        a            
where η is viscosity, η0 the viscosity at low strain rate (Newtonian viscosity), and the other constants, a, τF and n, are characteristic parameters of the pseudo-plasticity of the melt, obtained by curve-fitting via non-linear regression experimental data of Log(η) vs ω or dγ/dt. At high strain rate, the Carreau equation resumes to the simpler Power Law equation:η=m({dot over (γ)})n−1
n is called the power law index, and m is the consistency index, which can be expressed as a function of n, η0 and τF. The power law index is 1 for Newtonian fluids and smaller than 1 for pseudo-plastic fluids, i.e. fluids for which viscosity decreases with strain rate, a phenomenon known as “shear-thinning”; this is the case for polymeric melts. Polymeric melts with lower power law index require less torque to shear, everything else being equal.
Shear thinning of plastic materials is known to processors and is used practically to lower the viscosity of melts during the filling stage of injection molding by increasing the speed of the injecting piston. This is particularly useful in the case of thin wall injection molding where considerable forces are required to fill the mold when the viscosity of the melt remains quasi-Newtonian. In summary, shear-thinning is well defined analytically and used practically.
More recently, rheologists have started to analyze the effect of strain rate of a melt submitted to elongational flow, and defined the elongational viscosity. The analogy with shear viscosity behavior covers many areas, in the linear viscoelastic range, including strain rate temperature superposition effects. However, a remarkable difference of behavior is observed at large strain rate, in the non linear viscoelastic range: strain hardening results from increasing the elongational strain rate. This means that the elongational viscosity of a melt subjected to fast extensional flow increases, in contrast to the shear-thinning response of a fastly shear-deformed melt, for which viscosity decreases.
an oscillation is imparted to the melt, leading to the knowledge of either the steady shear viscosity or the complex viscosity, η*. It is well known to rheologists that plots of the complex viscosity, η*, versus ω, the angular frequency, are similar to plots of viscosity versus shear rate, the so-called Cox Merz's rule.
It is also known that shear thinning can be obtained, at a given temperature, by either increasing the shear rate or the frequency of oscillation of the melt at constant amplitude of oscillation. For example, the viscosity of PMMA at 239° C. can be reduced from 13,000 Pa-s to 2,000 Pa-s, i.e. by more than factor of 6, when the melt oscillates in shear at relatively low radial frequency, ω=100 radians seconds−1 (16 Hz).
Although it is known that the viscosity of a plastic melt can be reduced by shear thinning induced by vibration. In the linear viscosity range, at low strain amplitude, the viscosity reduction is instantaneous and only prevails under vibration, i.e. it ceases if the vibration ceases. In other words, the viscosity reduction induced by shear thinning is not preserved and the melt returns instantaneously to the Newtonian viscosity after the vibration ceases. Therefore, in the linear viscoelastic range, the viscosity reduction induced by vibration-shear thinning is required to be done while the material is injected or extruded, that is to say while the part is being shaped in a mold or a die. This implies the implementation of sophisticated vibration machinery added to traditional injection molding, blow molding or extrusion machines. Examples of such devices are described in other patents and applications (see for example; J. P. Ibar, U.S. Pat. No. 4,469,649 (1984), “Method and Apparatus For Transforming The Physical Characteristics of Material By Controlling The Influence of Rheological Parameters.”, J. P. Ibar, EP Patent 0 273 830 B1 (1991), “Method and Plant For Fabricating A Product By Injecting Material Into A Mold With Treatment of Injected Material.”, J. P. Ibar, U.S. patent application Ser. No. 07/882,754 (1990) “Method For Blow Molding Hollow Articles of a Synthetic Material” and U.S. Pat. Nos. 5,326,393 and 5,271,876, J. P. Ibar, U.S. Pat. No. 4,919,870 (1988), “Process of and Apparatus For Treating A Shaped Product”, J. P. Ibar, U.S. patent application Ser. No. 07/880,926 (1993), “Molding Deformable Materials With Use of Vibrating Wall Surfaces” and U.S. Pat. No. 5,306,129, J. P. Ibar, U.S. patent application Ser. No. 08/124,147 (1993), “Molding Apparatus and a Method of Using the Same”, J. P. Ibar, U.S. patent application Ser. No. 08/138,673 (1993), “Improved Injection Molding Process and Apparatus” and U.S. Pat. No. 5,494,426, J. P. Ibar, CA Patent 1,313,840 (1993) “Process and Device for Producing an Article by Injection of Material Into a Mold.”, and J. P. Ibar, EP Patent 0 274 317 (1993), “Process and Device for Extruding a Product in the Form of a Film, Plate, Tube, Bar or Thread.”). The same arguments can be said about the modification of the elasticity of a melt, which can be brought upon either by an increase of molecular weight or by melt vibration. The excess elasticity at a given temperature induced by the vibration condition ceases upon interruption of the vibration.
At larger strain amplitude of oscillation, in the non linear viscoelastic range, non linear effects induced by higher strain amplitude trigger a time dependence of rheological parameters and this effect is taken advantage of in U.S. Pat. Nos. 5,885,496 and 6,210,030 both to Ibar and both incorporated herein by reference in their entirety.
The present inventor has now discovered that it is possible to affect and control the rheological properties of a polymer melt by applying a longitudinal oscillation or vibration to the melt, in particular in a melt extensional situation. The values of the Carreau parameters and power law index can be adjusted thereby. The present invention is therefore a method to modify the value of at least one of the flow characteristic parameters, to render the melt, at will, either more pseudo-plastic, less pseudo-plastic, more viscous, more elastic, less viscous, less elastic, more strain hardening, less strain hardening, at least for a certain time. This method has utility for polymer processors who wish to control the processing characteristics or end use properties of the polymers without resorting to making changes in molecular structure of the polymer.